I'm in a bit of a hurry but I can list a few things you can do.
First SHADOW BOX.
When you have nothing to do, laying on the couch or at the grocery store start putting together some combos.
If you don't have access to a bag this can really help you! ^^
Also if you not only want to increase punching speed but a few other fundamentals that use your hands try googling a few reaction time games to play by yourself or with some friends, though some may sound stupid they can be very benifical. Irregular training can do amazing things!
Hope I helped a little!